The present invention relates to high purity ytterbium (Yb), a sputtering target made of high purity ytterbium, a thin film containing high purity ytterbium, and a method of producing high purity ytterbium.
Ytterbium (Yb) is a rare-earth element, and is contained in the earth's crust as a mixed composite oxide mineral resource. Rare-earth elements are named after because they are separated from relatively rare minerals, but they are not that rare in light of the overall earth's crust.
Ytterbium is a gray metal having an atomic number of 70 and an atomic weight of 173.0, and comprises a cubic close-packed structure at normal temperature. Ytterbium has a melting point of 819° C., boiling point of 1194° C., density of 6.97 g/cm3, its surface is oxidized in the atmosphere, and is also soluble in acid. With rare-earth elements, compounds having an oxidation number of 3 are generally stable, and ytterbium is also trivalent, but certain rare-earth elements are bivalent. The present invention covers all of the above.
In recent years, research and development for using ytterbium as a metal gate material and an electronic material for High-k use are being promoted, and ytterbium is a metal that is catching attention.
Nevertheless, ytterbium metal is a material which has a high vapor pressure and is hard to refine, and contains large amounts of volatile elements and the like as impurities. Accordingly, there is a problem in that it is not possible to adopt the process of refining ytterbium in a vacuum as with ordinary metal refining.
Conventionally, since no consideration was given to using ytterbium as an electronic component, documents related to the practical extraction method of ytterbium are limited. Only certain references were made as a part of extraction of rare earth metals, and these are introduced below.
In a method of producing the rare earth elements of Sm, Eu, Yb by mixing the oxide powders of Sm, Eu, Yb with misch metal to obtain a briquette shape, and reducing this with the vacuum thermal reduction method with misch metal as the reducing agent, disclosed is technology for subjecting misch metal to hydrotreatment in advance to obtain powdered hydrogenated misch metal, mixing this to obtain a briquette shape, and thereby preventing the oxidation/combustion during the granulation process of misch metal (for instance, refer to Patent Document 1).
In the foregoing case, although there is devisal in the use of the misch metal as the reducing agent, there is contamination from the misch metal and atmospheric contamination in the production process, and there is a problem in that high purification is difficult.
Additionally proposed is technology for reducing halide as a rare earth with calcium or calcium hydride, and, upon separating the obtained rare earth metal and the slag, placing a slag separating jib in the melted slag, solidifying the slag, and eliminating the slag together with the jig.
As the rare earths, lanthanum, cerium, praseodymium, and neodymium are selected (for example, refer to Patent Document 2). Nevertheless, since this technology is unable to sufficiently eliminate the slag, there is a problem in that it is difficult to achieve high purity.
Additionally disclosed is a method of producing rare earth metals based on the thermal reduction method of adding a reducing agent to the fluoride raw material as the rare earth metal and heating this at high temperature, wherein proposed is using a mixed composition of fluoride as the rare earth metal and lithium fluoride as the raw material, or adding one or more types of barium fluoride and calcium fluoride thereto. In the foregoing case, proposed is the use of a fused salt electrolytic solvent bath, and it is disclosed that the oxygen content will thereby 1000 ppm (for example, refer to Patent Document 3).
This technology is based on the use of a fused salt electrolytic bath, and there are problems in that it requires a complicated process and the effect of eliminating oxygen is insufficient. Moreover, there is a problem in that the used lithium, barium, calcium and the like will be accompanied as impurities.
Additionally proposed is a process of mixing a mixed composition of fluoride as the rare earth metal and lithium fluoride or a mixed composition in which one or more types of barium fluoride and calcium fluoride is added thereto, and rare earth metals, and extracting the rare earths based on heating and melting, wherein commercially available thermally reduced rare earths are used as the foregoing rare earths, and fused salt electrolysis for producing an alloy of the rare earth metals and iron group transition metals as the mixed composition.
Further disclosed is that, consequently, it is possible to obtain high purity rare earth metals in which the oxygen content is 300 ppm or less, and having few impurities such as calcium, lithium, and fluorine (for example, refer to Patent Document 4). This technology is also based on the use of a fused salt electrolytic bath, and there are problems in that it requires a complicated process and the effect of eliminating oxygen is insufficient. Moreover, there is a problem in that the used lithium, barium, calcium and the like will be accompanied as impurities.
Additionally proposed is a refining method of obtaining high purity rare elements by adding Mg or Zn to Ta-containing rare earth metals as impurities, melting this in a crucible, subsequently solidifying this, eliminating the high Ta-containing portion existing at the bottom of the crucible, and distilling the low Ta-containing portion in a vacuum (for example, refer to Patent Document 5). Since there are problems in that the added metals will be accompanied as impurities and the elimination of Ta is insufficient, the level of high purification still remains low.
As shown in the foregoing Documents, the refining effect of ytterbium is not necessarily sufficient, and there is a problem in that documents that seek the reduction of oxygen are limited, and that, even if sought, the reduction is insufficient.
Moreover, technologies that use fused salt electrolysis entail complicated and costly processes, and there is also a problem in that the refining effect is insufficient. Thus, the current situation is that there is no technology that is capable of efficiently and stably producing high purity ytterbium which has a high vapor pressure and is hard to refine in a molten state.    [Patent Document 1] Publication of Japanese Unexamined Application No.S61-9533    [Patent Document 2] Publication of Japanese Unexamined Application No.S63-11628    [Patent Document 3] Publication of Japanese Unexamined Application No.H7-90410    [Patent Document 4] Publication of Japanese Unexamined Application No.H7-90411    [Patent Document 5] Publication of Japanese Unexamined Application No. H8-85833